The wait is over. On October 23, 2009, in Commonwealth v. Haag, Justice McCaffery wrote for a unanimous Court, overruling Nieves and its progeny. You must be CONVICTED of the first DUI before the arrest of the second, otherwise, you have two first DUIs.
As an aside for any legal scholars, the cases specifically overruled are Misner, Nieves and Stafford.
Cheers to the Supremes!